The present invention relates generally to screens which are usually installed in water intakes, whether these are river or sea water intakes, having a filter element for retaining debris and particles carried by water which is to be passed through the screen.
A screen of this kind is normally disposed downstream of a grid composed of bars placed a few centimeters apart and protecting the screen from debris of large dimensions. Such screens are disposed either singly or in association with one or more other screens of the same type at a screening station. The filter element, whose mesh provides openings of only a few millimeters, is movable so that it may be periodically freed of the debris and particles which, being of larger dimensions than those of its mesh, progressively obstruct all of its openings and therefore clog it.
The movable filter element may comprise a panel adapted to be raised in its own plane in alternation with another panel of the same type disposed parallel to it and at a distance from it.
Alternatively the movable filter element may comprise an endless loop of elongated cross-section forming a filter chain, or of circular or polygonal cross-section forming a filter drum.
Whatever the form of the filter element, the element passes cyclically from an immersed position, in which it is progressively charged with various debris and particles, to an emerged position in which it is subjected to the action of jets of water under pressure acting countercurrent in order to free it of particles and debris thus attached to its surface, and so to make it capable of functioning once again as a filter in the course of its subsequent immersion.
The particles and debris entrained by the washing water have up to the present time usually been collected entirely in a channel provided for the purpose, and discharged by the latter to a drain.
The fact that the debris and particles were of mineral and vegetable origin, or that they contained living elements, particularly fish, does not indeed appear to have been given attention up to the present time.
There is now a growing anxiety to protect nature, and in particular to protect elements living in an aqueous medium, and this causes a problem because the flow of water taken from rivers or the sea is increasing, particularly in view of the large volume of water required by nuclear power stations. Furthermore, there is a tendency to install water intakes for large flows on estuaries or at the seaside and this aggravates the problem, since living elements carried by the water in question do not have the advantage in such installations of the beneficial influence of a possible current limiting their entrainment, as is the case in certain water intakes on rivers which are rationally equipped for this purpose.
It now definitely appears that living elements, particularly fish, which are trapped and entrained by the filter element of a screen of a water intake of the kind discussed, are irremediably subjected to an often prolonged stay out of the water during the emergence phase of the filter element, to the action of jets of washing water applied to the latter, which usually is a powerful action, and to a consequent violent projection into the channel provided for the collection of all the particles and debris which temporarily clog the filter element in question.
Various devices have certainly already been proposed which are intended to equip a screening station for water intakes for safeguarding living elements, particularly fish, carried by the water passed through the screening station.
In these devices use is either made of special filters which by themselves ensure only that the fish are safeguarded without having any general screening action, or conventional screening filters obliging the fish to stay for a certain time out of the water.
The present invention has the general object of making it possible to achieve conjointly the desired screening of the water and the safeguarding of the fish.